The present invention relates to a torque limiter for use in office appliances and the like. More particularly, the present invention relates to a torque limiter which generates a torque stably without little fluctuation and lengthens the life of a bearing without being affected adversely by an environment having a high temperature and a high humidity in which it is difficult to lubricating the torque limiter.
The torque limiter is classified into two types. In one type, a binding force is applied to an inner ring in a radial direction to generate a torque. In the other type, a spring is used to slidably contact one friction plate with other friction plate, with the one friction plate being pressed against the other friction plate in a thrust direction to generate a torque. In both types, the torque is generated by a frictional force. Many prior arts of the torque limiter are known (for example, Japanese Patent Application Laid-Open Nos. 8-270675, and 7-301248, 6-235447 and Japanese Utility Model Laid-Open No.5-8062).
Lubricating oil or lubricating grease is used to prevent a wear from occurring between the inner ring of the torque limiter and the spring thereof or the friction plate and between the friction plates, an abnormal heat generation, and an abnormal sound generated by seizure. The inner ring of the torque limiter is made of sintered metal and impregnated with the lubricating oil to use the inner ring as a lubricating mechanism.
The lubricating oil and the lubricating grease for use in the torque limiter frequently contain mineral oil, an aromatic compound and ester as the base oil thereof and additives such as a wear-resistant agent added to the base oil in dependence on use. The torque limiter is required to have the performance of retaining and maintaining an oil film for a long time by preventing contact between metals and stabilizing friction coefficients thereof. In the torque limiter for use in a paper transport apparatus of a copying machine, a printer, and the like and a mechanism for tensioning a ribbon or a sheet, there is a demand for the development for a lubricating agent allowing a torque to fluctuate little and preventing sounds from being generated by contact between the metals.
There is a demand that office appliances such as the copying apparatus using the torque limiter can be reliably used in different environments. There is also a demand for the development of lubricating oil allowing the torque to fluctuate little and preventing sounds from being generated by the contact between the metals in an environment having a high temperature and a high humidity (for example, 40° C. and relative humidity (RH) in the neighborhood of 90%) environment where it is difficult to form an oil film.
Resin such as polycarbonate resin or ABS resin having a high processability is used for parts on the periphery of the torque limiter. Thus the parts made of the resin are cracked, broken or the surfaces thereof may become rough, i.e., a so-called chemical attacking phenomenon may occur owing to contact between the parts made of the resin and lubricating oil, for use in the torque limiter, which has leaked from the torque limiter or between the parts made of the resin and the vapor of the lubricating oil. For example, the lubricating oil using ester or an aromatic compound as its base oil has a high oil film-forming performance and satisfies the torque performance necessary for the torque limiter to perform. But the lubricating oil containing the base oil, whose molecules have aromatic rings and polar groups, as its main component is liable to chemically attack the parts made of the resin. The ester-containing base oil is apt to be hydrolyzed in an environment having a high temperature and a high humidity.
To prevent the lubricating oil from chemically attacking the members made of resin, conventionally known is a rust-proof oil containing the rust-proof agent and the antioxidant both added to the base oil, as disclosed in Japanese Patent Application Laid-Open No.2002-348688. The base oil contains polyolefin oil. The rust-proof agent is at least one kind of metallic salt selected from among metallic salts of sulfonic acid and metallic salts of monocarboxylic acid. The antioxidant is a phenolic antioxidant.
As the lubricating oil or the lubricating grease, for the torque limiter, which generates a torque which fluctuate little and allows the torque limiter to have a long life by preventing contacts of metals which occur owing to breakage of an oil film and which does not chemically attack resin, the lubricating oil impregnating a bearing therewith or the lubricating grease disclosed in Japanese Patent Application Laid-Open No.2002-249794 contains the base oil consisting of the compound of synthetic saturated hydrocarbon and at least one kind of phosphate ester selected from among aliphatic phosphate and aliphatic phosphite. The phosphate ester is contained at 1 to 8 wt % for the total weight of the lubricating oil.
Utilizing a torque generated by the torque limiter, the torque limiter is used as a part of a paper supply mechanism of a copying apparatus and a printer. In an environment having a high temperature and a high humidity, the viscosity of the lubricating oil lowers, the thickness of an oil film on members of the torque limiter decreases, and water in the air penetrates into the lubricating oil in the form of droplets and into the lubricating surface. Thereby the oil film is broken in a short period of time, thus causing the torque limiter to chatter (abnormal torque) and thus the paper supply function to deteriorate. The torque limiter is liable to chatter in an environment having a high humidity in the neighborhood of the dew point temperature.